1. Field of the Invention
The present invention relates to a liquid thermosetting sealing agent for a polymer electrode membrane fuel cell composed of membrane electrode assembly having separators laminated thereon, a single cell of a polymer electrode membrane fuel cell and a polymer electrode membrane fuel cell having a seal formed by the liquid thermosetting the sealing agent, a process for producing or regenerating them.
2. Description of Related Arts
A fuel cell system is a power generation system mainly due to a fuel cell in which hydrogen as a fuel gas is supplied to the side of a hydrogen pole of the fuel cell, while oxygen as an oxidant gas is supplied to the side of an oxygen pole of the fuel cell to generate electric power. The fuel cell system, which directly converts chemical energy into electric energy, has been recently attracted from the viewpoints of high power generation efficiency and very little exhaust of harmful substances.
Configuration of the general fuel cell system will now be described by referring to FIGS. 1 and 2.
As shown in FIGS. 1 and 2, a fuel cell FC is composed of single cells C comprising separators 1 and 2 and a membrane electrode assembly MEA (hereinafter referred to as “MEA”, and is composed of a lamination of approximately 200 single cells C.
The separators 1 and 2 cut off the fuel gas, the oxidant gas and a coolant, and possesses passages PH and PO, respectively for introducing the fuel gas and the oxidant gas into the interior of the MEA, and a passage PW for introducing the cooling water into the MEA to allow the MEA to cool. On the other hand, an oxygen pole E0 (cathode) and a hydrogen pole EH (anode) are provided on the MEA. Also, diffusion layers D are provided on the MEA at both sides of the oxygen pole E0 and the hydrogen pole EH. The MEA is thus configured.
As the material for the electrolyte membrane M, for example, a perfluorocarbon sulfonic acid membrane, which is a proton-exchanging membrane, has been used. This electrolyte membrane has a plurality of proton-exchanging groups in the solid macromolecule, exhibits a proton specific resistance as low as approximately 20 Ω/cm proton and, thus, acts as a proton-conductive electrolyte. As described above, because of using the electrolyte membrane M, the fuel cell FC is referred to as a polymer electrode membrane fuel cell.
A catalyst contained in the oxygen pole EO is a catalyst which produces oxygen ions form oxygen, while a catalyst contained in the hydrogen pole EH is a catalyst which produces protons from hydrogen.
Outside the oxygen pole EO, a gas passage PO at the oxygen pole side which allows a supply gas as the oxidant gas for flowing into the oxygen pole EO, is provided, and outside of the oxygen pole EH, a gas passage PH at the hydrogen pole side which allows a supply hydrogen H2 as the fuel gas for flowing into the hydrogen pole EH, is provided. The inlet and the outlet of the gas passage PO at the oxygen pole side are connected to an air-supplying apparatus (not shown), and the inlet and the outlet of the gas passage PH at the hydrogen pole side are connected to a hydrogen-supplying apparatus (not shown)
The diffusion layers D are each provided in contact with the gas passage PO and the gas passage PH on the surface of the separators 1 and 2, respectively, has a function of transmitting the electrons between the electrode EH and the separator 2, and the function of diffusing the fuel gas or the oxidant gas, and generally is composed of carbon fibers.
The separators 1 and 2 each is composed of carbon type materials, has a function of separating the fuel gas and the oxidant gas from the coolant gas, respectively, and possesses a passage PW as well as the above passages PH and PO, and has an electron-transmitting function.
In this fuel cell FC, when the supply air is supplied into the gas passage PO at the oxygen pole side and the fuel gas is supplied into the gas passage PH at the hydrogen pole side, the hydrogen is ionized into protons by the catalytic function of the catalyst at the hydrogen pole EP, and the produced protons are migrated in the electrolyte membrane M to arrive at the oxygen pole EO. The protons arriving at the oxygen pole EO are readily reacted with oxygen ions produced from the oxygen contained in the supply air to form water. The supply air containing the water thus produced and the oxygen remaining unused is discharged from the outlet of the fuel cell FC at the oxygen pole side as an exhaust gas (the exhaust gas having a large moisture content). Electrons e−, which are produced during the ionization of the hydrogen, reach the oxygen pole EO via an external load such as a motor.
In the fuel cell FC having such a configuration, the fuel gas, the supply gas and the coolant are supplied into each single cell via dependent passages PH, PO, and PW, respectively. In order to partition each system in a gas sealed manner, a technique for applying a seal q1 becomes important.
Although the portions where the seal q1 should be applied somewhat depends upon the construction of the fuel cell FC, for example, in the case of the fuel cell shown in FIGS. 1 and 2, there can be mentioned portions around the communication ports pierced through the fuel cell for the purpose of supplying the fuel cell, the humidified water, and the coolant into respective single cells C, an outer circumference of the membrane electrode assembly MEA, an outer circumference of the surface of the coolant passage allowing the coolant for flowing along the surface of the separator in order to cool the surface of the separator, and outer circumferences of the separators 1 and 2, such as front and rear surfaces thereof.
As the sealing technique for gas-tightly sealing such portions, a technique in which a laminate comprising sheets or O-rings composed of a fluorine- or silicone-series organic rubber compressed and laminated with a liquid curing material which is adhered and cured is used to apply a seal due to the resilient force of the rubber, a technique in which graphite or any other ceramic fiber sheets are compressed and the compressed material is sealed in a state of left fastening, or a mechanical sealing techniques using a caulking, etc. has been conventionally utilized.
If the fuel cell is a portable type or is carried on a vehicle, etc., the fuel cell is required to have a restricted size. Particularly, the single cell making up the fuel cell FC is required to be thin. For this reason, it is an important how to produce the membrane electrode assembly MEA and the separators 1 and 2 in a thinner form particularly, since the separators 1 and 2, which are directly in contact with the seal q1, are mainly composed of brittle carbon type materials, they are easily fractured at the time of the lamination when they are configured to be a thin form.
For this reason, it is the present situation that a soft sealing material having an adequate resilient force such as an organic rubber is used to apply the seal q1.
Recently, the sealing agent for applying the seal q1 to the single cell C of the fuel cell has been developed (see WO99/53559). According to this publication, the sealing agent comprises an oligomer having prescribed addition-polymerization properties, a curing agent having at least two hydrogen atoms bonded to silicon atom, and a hydrosilylation catalyst.
However, when a seal is applied to the single cell of the fuel cell utilizing any of the conventional techniques, there poses the following problems:
(1) In the operation of the fuel cell FC, the atmosphere at the power generation portion is at a high temperature and under a highly acidic condition and the coolant is flowing at the cooling portion; thus, the seal is required to have durability, which can separate such severe atmospheres from each other as a rule. However, if the conventional adhesive is used to apply a seal (if a seal is formed by the intervention of organic rubber or by using an adhesive), it is difficult to secure the durability over a prolonged period of time. Particularly, only by the function of the adhesion property due to the sealing agent it is difficult to secure the durability under these atmospheres over a prolonged period of time.
For this reason, it is desired that the surface pressure during the lamination of the single cells is kept substantially constant to secure the sealing properties and the difference in the stresses is decreased to prevent the fracture.
(2) In the fuel cell FC composed of laminating the separators 1 and 2 and the membrane electrode assembly MEA, when the normal thermosetting adhesive is intervened between the laminates among the separators 1 and 2 and the membrane electrode assembly MEA to form seals, the thermosetting sealing agent for a polymer electrode membrane fuel cell, the membrane electrode assembly MEA, and the separator 1 are exposed to the same heating conditions. However, since the electrolyte membrane M of the membrane electrode assembly MBA has a temperature which decreases a kinetic viscosity-elastic modulus as low as from 75 to 110° C., the electrolyte membrane M is unduly crept when being exposed under such conditions over a prolonged period of time to decrease the mechanical durability. Accordingly, it is desired to thermally cure the thermosetting sealing agent for a polymer electrode membrane fuel cell at a temperature as low as possible for a minimized period.
A typical example of sealing agent, which has been commonly utilized, includes a fluorine-series sealing agent, which can be used at a high temperature, at a high humidity, and under a strongly acidic conditions. The curing temperature of this sealing agent is not less than 150° C. If this sealing agent is merely thermally cured at a temperature lower than the appropriate temperature as disclosed in WO99/53559, the durability of the resulting seal is decreased. For this reason, the development of the sealing techniques has been desired, which can impart sufficient durability to the resulting seal q1 without deteriorating the electrolyte membrane M.
(3) In the fuel cell composed of the lamination of the separator 1 with the membrane electrode assembly MEA, when the seal q1 is intervened between the lamination of the separator 1 and the membrane electrode assembly MEA, unevenness of the thickness of the membrane electrode assembly MEA causes unevenness in the gap of the seal. Due to the unevenness of each gap in the laminate state, difference in stresses is generated at the portion where the separators 1 and 2 are in contact with the seal q1. With regard to this difference in stresses, a separator produced by caving sintered graphite or a separator formed by mixing carbon powders with 15–30% of resin, which has low fracture stress, is easily fractured at the time of lamination and fastening.
For this reason, it has been desired to provide a process for the application of the seal in which difference in the stresses to unevenness of the gap of each seal is difficult to be brought about.
(4) The thermal strain and the seal stress (to the carbon type separator) vary depending upon the hardness of the seal q1 after curing. Specifically, a seal q1 having relatively low hardness, which is a raw material having low cross-linking degree, is easy to undergo thermal strain, but due to its low degree of increasing the counterforce against the amount of the compression, the difference between stress between the seals q1 in the laminated state of the single cell C becomes small.
In contrast, a seal q1 having relatively low hardness can decrease the thermal strain, but has large difference in the stresses in the laminated state of the single cell.
Consequently, it has been desired to provide a sealing agent for a polymer electrode membrane fuel cell having hardness with well-balanced properties between the two antimony properties and to provide a process for producing a polymer electrode membrane fuel cell FC utilizing such a sealing agent.
(5) Moreover, if the seal q1 is deteriorated or if any other abnormality of the seal q1 occurs, the fuel cell must be immediately recovered. Such recovery requires much more times and labors. Consequently, there is a demand to develop a process for easily recovering the fuel cell by applying a seal q1 having high durability under the utilization of the fuel cell.